The present invention relates generally to x-ray detectors, and in particular, to a solid-state semiconductor detector such as cadmium zinc telluride (CZT) used for quantitative x-ray imaging.
Measurements of the x-ray absorption by an object, for example, at two different x-ray energies, can reveal information about the composition of that object as decomposed into two selected basis materials. In the medical area, the selected basis materials are frequently bone and soft tissue. The ability to distinguish bone from soft tissue allows x-ray images to yield quantitative information about in vivo bone density for the diagnosis of osteoporosis and other bone disease.
Alternatively, the selection of other basis materials allows dual energy x-ray measurements to be used for the analysis of body composition by distinguishing between fat and non-fat tissue, or for baggage scanning by distinguishing between explosive and non-explosive materials.
High resistivity solid-state semiconductors such as cadmium zinc telluride (CZT) may be used to detect x-rays passing through a measured object in a single or dual energy x-ray system. In a planar-contact CZT detector, a voltage is imposed between an anode and cathode positioned on opposite faces of a CZT crystal. X-rays pass through the anode into the crystal to release electrons that are attracted to the cathode. The number of released electrons is proportional to the photon energy allowing high and low energy x-ray photons to be distinguished by pulse height.
Detectors of this design can exhibit a variation in the energy measurement dependent on the location of the x-ray radiation interaction within the crystal. This measurement variation results in a reduction in accuracy as well as poor energy resolution. The prior art has addressed this measurement variation by reducing the anode size and attaching “steering electrodes” to the crystal surrounding the anode. The steering electrodes are operated at an intermediate voltage between the voltages of the anode and cathode to shape the electrical field within the detector improving charge collection.